Neutrino mass ordering from the next Galactic supernova at DUNE, HK, and JUNO

This paper demonstrates that combining the electron neutrino neutronization burst and the accretion-phase rise-time of electron antineutrinos from a future Galactic supernova will enable the DUNE, Hyper-Kamiokande, and JUNO detectors to definitively determine the neutrino mass ordering with high statistical significance.

The Gist

Imagine the universe is holding a massive, cosmic drumbeat. When a massive star dies and collapses in on itself, it doesn't just go quiet; it screams in a language we can barely hear: neutrinos. These are ghostly, tiny particles that zip through everything, including the Earth, without stopping.

This paper is like a detective's guidebook for the next time a star explodes in our own galaxy (a "Galactic Supernova"). The authors are asking: Can our new, giant neutrino detectors "hear" this explosion well enough to solve a 50-year-old mystery about the weight of these ghost particles?

Here is the breakdown of their investigation, using simple analogies.

The Mystery: The "Weight" of Ghosts

Neutrinos come in three "flavors" (like ice cream flavors: vanilla, chocolate, and strawberry). Scientists know these flavors can change into one another as they travel, a bit like a chameleon changing colors. However, there is a fundamental question: Which flavor is the heaviest?

There are two main theories about how they are ordered by weight:

1. Normal Ordering (NO): Like a pyramid, where the lightest ones are at the bottom.
2. Inverted Ordering (IO): Like an upside-down pyramid, where the heaviest ones are at the bottom.

The paper argues that the next supernova explosion will be the perfect test to figure out which pyramid is the real one.

The Two Clues: The "Flash" and the "Ramp"

The authors focus on two specific moments during the explosion, which act like two different clues.

Clue 1: The Neutronization Burst (The "Flashbulb")

• What happens: When the star's core first bounces back, it creates a massive, sharp spike of electron-neutrinos (the "vanilla" flavor) that lasts only about 20–30 milliseconds. It's like a camera flash going off for a split second.
• The Detective Work:
  • If the universe is Inverted (IO), this flash of vanilla neutrinos will appear clearly at our detectors.
  • If the universe is Normal (NO), this flash gets "swapped" with other flavors on its way to Earth and disappears.
• The Result: The authors found that the DUNE detector (a giant tank of liquid argon) is like a super-sensitive camera. It will see this flash so clearly that it can tell the difference between the two theories with 99.9999% certainty (6-sigma confidence). Hyper-Kamiokande (HK) is also very good at this, though slightly less sensitive than DUNE.
• The Good News: This clue is very robust. It doesn't matter what kind of star exploded (whether it was a heavy star or a lighter one); the flash behaves the same way. It's a "standard candle" for the universe.

Clue 2: The Rise-Time (The "Ramp")

• What happens: A few moments after the flash, the star enters an "accretion phase." Here, the star is still feeding material into the core. During this time, the "heavy" neutrinos (muon and tau flavors) start rising in number much faster than the electron-antineutrinos.
• The Detective Work:
  • If the universe is Inverted (IO), the electron-antineutrinos we detect will rise in number very quickly (a steep ramp).
  • If the universe is Normal (NO), they rise slower (a gentle slope).
• The Problem: This clue is tricky. The shape of the ramp depends heavily on the specific details of the exploding star. It's like trying to guess the weight of a person by how fast they run, but you don't know if they are running on sand, mud, or ice. Different stars (different "progenitors") create different ramps, which can confuse the detectors.
• The Solution: To fix this confusion, the authors invented a new math trick. Instead of looking at the whole ramp, they looked at a ratio: "How many particles did we see at 20 milliseconds compared to 100 milliseconds?"
  • This ratio acts like a filter, canceling out the confusion caused by different star types.
• The Result: Using this ratio, HK and JUNO (a detector in China) can still distinguish the theories, though with less certainty than the "Flash" clue. HK can do it with high confidence, while JUNO has a harder time because it is smaller and catches fewer particles.

The "Ghostly" Complication

There is one more twist. The authors considered a scenario called Flavor Equalization (FE). Imagine if, deep inside the star, the neutrinos started talking to each other so much that they all mixed up perfectly, becoming a uniform soup.

• If this happens, the "Ramp" clue gets muddy. The steep ramp of the Inverted theory and the gentle ramp of the Normal theory get squashed into a middle-ground shape.
• The authors found that while this makes the "Ramp" clue harder to read, the "Flash" clue remains safe because the conditions inside the star during the flash prevent this mixing from happening.

The Verdict

The paper concludes that the next Galactic supernova will be a golden opportunity.

1. DUNE will likely solve the mystery immediately by watching the Flash (Neutronization Burst).
2. HK and JUNO will help confirm this by analyzing the Ramp (Rise-time), especially if they use the new "Ratio" math trick to filter out the noise.

By combining the data from these different detectors and looking at both the Flash and the Ramp, scientists will finally be able to definitively answer the question: Is the neutrino weight pyramid Normal or Inverted?

The paper does not claim this will help with medical treatments or energy production; it is purely about solving a fundamental puzzle of how the universe works.

Technical Summary

Technical Summary: Neutrino Mass Ordering from the Next Galactic Supernova at DUNE, HK, and JUNO

Problem Statement
The neutrino mass ordering (MO)—whether the neutrino mass eigenstates follow a normal ordering (NO, $\Delta m^2_{atm} > 0$) or inverted ordering (IO, $\Delta m^2_{atm} < 0$)—remains a fundamental open question in neutrino physics. While long-baseline accelerator experiments are probing this, a core-collapse supernova (CCSN) in our Galaxy offers a unique, high-statistics opportunity to determine the MO through flavor-dependent neutrino emission. However, extracting the MO from supernova neutrinos is complicated by uncertainties in the progenitor star models and the complex hydrodynamics of the supernova core, which influence the initial neutrino flux. Furthermore, collective neutrino oscillations (specifically flavor equalization, FE) occurring deep within the star could obscure standard MSW oscillation signatures. This paper addresses the challenge of distinguishing NO from IO in the presence of progenitor model degeneracies and potential collective oscillation effects using the next generation of neutrino detectors.

Methodology
The authors perform a comprehensive statistical analysis of neutrino signals from a Galactic CCSN located at a distance of 10 kpc. The study utilizes:

• Source Models: Six realistic CCSN hydrodynamic simulations from the Garching group, corresponding to progenitor masses of 12, 15, 17.6, 20, 25, and 27 $M_\odot$. These provide time-dependent luminosities and average energies for electron neutrinos ($\nu_e$), electron antineutrinos ($\bar{\nu}_e$), and heavy-lepton flavors ($\nu_x$).
• Oscillation Scenarios: Three scenarios are considered: Normal Ordering (NO), Inverted Ordering (IO), and Flavor Equalization (FE). The FE scenario, arising from fast collective oscillations, is treated as an intermediate case where flavor ratios are equalized upon escape from the core. The authors note that FE is suppressed during the neutronization burst due to high electron degeneracy but may be active during the accretion phase.
• Detectors: The analysis focuses on three detectors:
  • DUNE (Deep Underground Neutrino Experiment): A 40 kt Liquid Argon TPC, sensitive primarily to $\nu_e$ via charged-current interactions.
  • Hyper-Kamiokande (HK): A 187 kt Water Cherenkov detector, sensitive to $\bar{\nu}_e$ via Inverse Beta Decay (IBD) and $\nu_e$ via elastic scattering.
  • JUNO (Jiangmen Underground Neutrino Observatory): A 20 kt Liquid Scintillator detector, sensitive to $\bar{\nu}_e$ via IBD.
• Simulation Tools: Event rates are computed using the SNOwGLoBES framework, fed with oscillated fluxes derived from the Garching simulations and the specific oscillation scenarios.
• Statistical Analysis: A $\chi^2$ analysis is employed to quantify the probability of misidentifying the MO ($P_{mis}$). The study compares "test" scenarios against "true" scenarios across all combinations of progenitor masses and oscillation scenarios. Two distinct observables are analyzed:
  1. Neutronization Burst: The sharp $\sim$20–30 ms peak in $\nu_e$ flux.
  2. Accretion Phase Rise-Time: The time evolution of the $\bar{\nu}_e$ flux. To mitigate progenitor degeneracies, the authors propose using cumulative event counts and a ratio index defined as $R = N(20 \text{ ms}) / N(100 \text{ ms})$.

Key Contributions and Results

1. Neutronization Burst Sensitivity:

   • The $\nu_e$ neutronization burst provides a robust, largely model-independent signature. In the IO scenario, the burst peak appears (as $\nu_e$ flux retains a component of the original $\nu_e$), whereas in NO, the peak is suppressed (as $\nu_e$ flux is dominated by $\nu_x$).
   • DUNE achieves a discrimination sensitivity of $\gtrsim 6\sigma$ for all progenitor models, with a misidentification probability $< 10^{-10}$.
   • HK achieves a sensitivity of $\gtrsim 4\sigma$ (misidentification probability $< 10^{-6}$).
   • The analysis confirms that the burst signature is largely independent of the progenitor mass model and the FE scenario (which is suppressed in this phase).

2. Accretion Phase Rise-Time Sensitivity:

   • The rise-time of the $\bar{\nu}_e$ flux is sensitive to MO because the $\nu_x$ luminosity rises faster than $\bar{\nu}_e$. In IO, the $\bar{\nu}_e$ flux (dominated by $\nu_x$) rises faster than in NO.
   • However, simple bin-by-bin event analysis suffers from significant degeneracies between different progenitor masses and the inclusion of the FE scenario.
   • Improved Observables: The authors demonstrate that using cumulative event counts and, more effectively, the ratio index ($N_{20ms}/N_{100ms}$) significantly reduces statistical fluctuations and progenitor model dependence.
   • HK Results: Using the ratio index, HK achieves $\sim 5\sigma$ discrimination for 91–96% of model combinations.
   • JUNO Results: JUNO achieves $\sim 3\sigma$ discrimination for 75% of models using the ratio index, limited by its smaller fiducial volume and lower event statistics compared to HK.
   • The presence of the FE scenario reduces the statistical significance of the rise-time analysis compared to the neutronization burst, as FE produces an intermediate rise-time behavior.

Significance and Claims
The paper claims that the next Galactic supernova will offer a definitive opportunity to determine the neutrino mass ordering, provided that data from multiple detectors and observables are combined.

• Complementarity: The neutronization burst serves as a "standard candle" for MO determination, offering high confidence and model independence, particularly for DUNE and HK. The accretion phase rise-time, while more susceptible to model uncertainties and collective oscillation effects, provides crucial complementary information.
• Robustness against Uncertainties: The study highlights that while progenitor mass uncertainties and collective oscillations (FE) pose challenges for the accretion phase analysis, the use of ratio-based observables and cumulative statistics can mitigate these issues to a significant degree.
• Detector Synergy: The combination of DUNE's sensitivity to $\nu_e$ (burst) and HK/JUNO's sensitivity to $\bar{\nu}_e$ (rise-time) is essential. The authors conclude that a definitive determination of the MO will likely require the combined analysis of both the neutronization burst and the accretion phase information across these detectors.

The authors remain modest regarding the FE scenario, noting that it remains an active area of research with uncertain theoretical predictions. They emphasize that future improvements in supernova simulations and a deeper understanding of collective oscillations will be necessary to further refine the constraints on misidentification probabilities.